The invention relates to a method for operating a system for the steam reforming of a hydrocarbon in particular to a method for operating a mobile system for the steam reforming of methanol in a fuel-cell-operated motor vehicle for supplying the hydrogen required for the fuel cells. The term xe2x80x9chydrocarbonxe2x80x9d also refers here, for the sake of simplicity, to hydrocarbon derivatives, such as, for example, methanol.
As is known, the steam reforming reaction for the reforming of, for example, methanol takes place endothermally and is carried out at a reaction temperature which is higher than room temperature. When the system is cold-started, therefore, hydrogen cannot be supplied immediately by means of the steam reforming reaction, but, instead, the system parts first have to be brought to a corresponding operating temperature. It is precisely in the application of the system to motor vehicles, however, that it is desirable, after an operation to start the vehicle and therefore also the reforming system has been triggered, to have the drive power furnished by the fuel cells available as soon as possible, which in turn makes it necessary for the reforming system to be capable of supplying hydrogen as quickly as possible. Different procedures for cold-starting systems of this type have already been proposed.
Thus, it is known from the patent specification U.S. Pat. No. 4,820,594, in a reforming reactor housing, to provide, in addition to the actual reforming reactor part, a combustion part, to which a combustible hydrocarbon/air mixture is delivered in a first operating phase during the cold-starting of the system, the said mixture being burnt there with an open flame and thereby heating the reforming reactor part located above it. The reforming reaction is then started after a suitable temperature has been reached.
In a system for the steam reforming of a hydrocarbon, as described in the patent specification U.S. Pat. No. 5,110,559, the reforming reactor housing is likewise divided into a burner part and a reforming reactor part, in order, during cold-starting, to heat the reforming reactor part by means of the burner part. For this purpose, a combustible mixture emanating from the reforming reactor is ignited in the burner part, the combustible hydrocarbon to be reformed being delivered to the reforming reactor even during cold-starting. The hot combustion exhaust gases are transferred from the reforming reactor into a downstream CO shift converter, in order consequently to heat up the latter and thereby bring the system to operating temperature more quickly.
It is known from the patent specification DE 44 23 587 C2 that in a reforming reactor filled with suitable catalyst material, for example Cu/ZnO material, depending on the control of the delivery of the individual reaction partners into the reactor and on the temperature prevailing there, hydrogen can be obtained optionally by means of partial oxidation, which takes place exothermally, and/or endothermal steam reforming of methanol. With suitable process management, the two reactions proceed in parallel, and an autothermal reaction sequence can be established. It is also known from the patent specifications FR 1,417,757 and FR 1,417,758 mentioned there that, during a cold-start of a system for the steam reforming of methanol, a mixture of methanol and oxidizing agent can first of all be introduced into the reforming reactor in order to cause a corresponding combustion reaction to take place there and consequently heat up the reactor. The delivery of the oxidizing agent is then terminated and, instead, the methanol/steam mixture to be reformed is delivered and the steam reforming reaction started. In the systems there, the hydrogen is supplied by means of selective hydrogen separation via hydrogen-permeable membrane walls which are integrated into the reactor.
The preliminary publication EP 0 217 532 A1 describes a reactor for the partial oxidation of methanol, which has a copper-containing catalyst in an upstream zone and a catalyst from the platinum element group in a downstream zone. During cold-starting, the delivered mixture of methanol and of an oxygen-containing gas passes through the upstream zone to the downstream zone, where spontaneous methanol oxidation occurs, which leads to an increase in temperature up to a value at which partial methanol oxidation develops in the upstream zone, specifically in the manner of a xe2x80x9chot spotxe2x80x9d.
The preliminary publication WO 96/00186 A1 describes a self-starting hydrogen generation system with a reactor for methanol conversion, which likewise contains, on the one hand, a copper-containing material and, on the other hand, a metal of the platinum element group as catalyst material. In the warmed-up operating mode, a self-maintaining partial methanol oxidation reaction takes place, and, by means of the copper-containing catalyst, an ignition of a methanol oxidation reaction is said to be achieved even at room temperature. Moreover, in the warmed-up operating mode, methanol reforming may additionally be carried out by the delivery of water. In order to keep the carbon fraction in the product gas low, the product gas can be led via a selective CO oxidation catalyst or a CO shift catalyst.
The preliminary publication JP 63-129002 (A) discloses a reactor for methanol conversion which has, in succession in the direction of flow, a Pd-based combustion catalyst zone, a Znxe2x80x94Cr reformer catalyst zone and a Cuxe2x80x94Zn reformer catalyst zone and to which a methanol/water mixture and also air are delivered. In the reformer catalyst region, a reforming reaction is carried out, with heat being delivered from the adjacent combustion catalyst bed.
The technical problem on which the invention is based is to provide a method of the type mentioned in the introduction, by means of which a system for the steam reforming of a hydrocarbon can be brought to its operating temperature as quickly as possible and hydrogen can thereby be supplied correspondingly quickly.
The invention solves this problem by providing a method wherein, at least part of the reforming reactor is used as a multi-function reactor unit, specifically in cold-starting, during a first operating phase, in a first function as a catalytic burner unit and, during a subsequent second operating phase, in a second function as a so-called POX unit, that is to say a unit for the partial oxidation of the hydrocarbon delivered.
The combustion heat generated as a result of catalytic combustion in the first operating phase in this multi-function reactor unit serves for heating at least one downstream system unit, for example a following part of the reforming reactor and/or a following CO oxidizer, and is transported there by the hot combustion exhaust gas and/or by heat conduction. The initial function as a catalytic burner unit ensures rapid first heating of the system. The partial oxidation of the hydrocarbon taking place in the subsequent second operating phase proceeds exothermally and therefore generates further heat for heating the system. At the same time, in this operating phase, hydrogen is already being generated and is therefore available, for example, for the fuel cells of a motor vehicle, before the endothermal steam reforming reaction can then proceed and supplies further hydrogen after a corresponding operating temperature of the system has been reached. In this case, a partial oxidation reaction can take place simultaneously with the reforming reaction or alternately with the latter, for example in order to implement autothermal process management, if there is a need for this.
Furthermore, the method involves one or more of the following three measures. First, there may be provision, shortly before the transition from the first to the second operating phase, for adding water to the delivered mixture of fuel and oxygen-containing gas. Undesirable peak values of the carbon monoxide content in the product gas mixture during the transition to the second operating phase can thereby be avoided. Moreover, the added water can prevent overheating zones during catalytic combustion and function as a heat transfer medium in order to transport the combustion heat occurring during catalytic combustion further on into downstream system units. Secondly, there may be provision, during the first operating phase, for increasing the fuel flow rate with a rising temperature of the multi-function reactor unit, preferably in such a way that the fraction of unburnt fuel in the product gas still just remains desirably low. This means that the fuel flow rate is increased to the extent to which the oxidizing capacity of the multi-function reactor unit grows, with increasing temperature, as regards its catalytic burner function. Thirdly, there may be provision for setting the flow rate of oxygen-containing gas substoichiometrically as early as during the first operating phase, thus resulting, in the product gas, in cleavage products, such as hydrogen and the like, due to thermal decomposition of the fuel used, if appropriate in interaction with added water or water occurring as a result of combustion. These cleavage products can be burnt by means of the renewed addition of air and can thus be utilized for heating downstream components of the reforming system.
By virtue of this mode of operation according to the invention, the system, which, for example, is installed in a mobile manner in a fuel-cell-operated motor vehicle, is capable of delivering hydrogen very quickly after starting and of being heated up rapidly to the operating temperature necessary for the steam reforming reaction.
In a method according to the present invention, that part of the reforming reactor which is used as a multi-function reactor unit serves after the end of a cold-starting phase, during subsequent normal operation when the system has warmed up, in a third function at least temporarily as a reformer unit for the steam reforming of the delivered hydrocarbon and/or as a CO shift unit for converting undesirable carbon monoxide into carbon dioxide.
In a method according to the present invention, there is a transition from the first to the second operating phase after only a few seconds, so that hydrogen is accordingly delivered in appreciable quantities after only a few seconds as a result of the partial oxidation reaction.
In a method according to the present invention, the hydrocarbon to be reformed or hydrogen is used as fuel for catalytic combustion in the multi-function reactor unit during the first operating phase. The advantage of this is that the fuel is readily available, since the hydrocarbon to be reformed is in any case stored in a reservoir, and the hydrogen is generated, for example, during a preceding active operation of the system and part of this hydrogen can be intermediately stored for this later use.
In a method according to the present invention, an inlet-side part of the reforming reactor serves as the multi-function reactor unit, whilst the remaining reforming reactor part functions during the second operating phase, at least in certain regions, as a post-reforming and CO shift converter stage. As a result, the hydrocarbon fractions which have possibly not been converted in the upstream multi-function reactor unit are fully converted in this outlet-side reforming reactor part and, at the same time, the carbon monoxide which has occurred during conversion in this reactor part and in the upstream multi-function reactor unit is converted into carbon dioxide by means of the so-called CO shift reaction with water. Too high a CO fraction in the product gas of the reforming reactor is undesirable in applications in connection with fuel cells, since the carbon monoxide acts in the fuel cells as a catalyst poison.
In a method according to the present invention, during the second operating phase water is added to a greater extent than during subsequent normal operation when the system has warmed up. This improves the transport of heat and reduces the CO fraction in the process gas, as compared with normal operation.
In a method according to the present invention, the substance mixture emerging from the reforming reactor is led through a downstream CO oxidizer, which is thus heated, as early as in the first operating phase, by the combustion gas from catalytic combustion in the multi-function reactor unit and is already performing essentially its normal function in the second operating phase. This normal function involves oxidizing carbon monoxide, which may possibly still be present in the substance mixture led through, so as to form carbon dioxide. In addition, in the first operating phase the CO oxidizer is heated by means of a specific catalytic combustion process. For this purpose, a fuel and an oxygen-containing gas stream are additionally introduced into the CO oxidizer.
In a method according to the present invention, a catalytic burner assigned to the reforming reactor and/or to an evaporator upstream of the latter is at least partially fed, starting from the second operating phase, with the hydrogen-containing product gas formed in the reforming reactor, the said product gas optionally being led first through intermediate system components, such as a CO oxidizer and the anode part of a fuel-cell system. If the continuously generated product gas is not sufficient as fuel, intermediately stored hydrogen or the hydrocarbon also used for reforming may be delivered as fuel to the catalytic burner.
Advantageous embodiments of the invention are described below with reference to the drawing.